Los costos de gas no se suman en un contrato de transacción simple

Tengo un contrato simple que ayuda a hacer un pago a otra cuenta. Digamos que comienzo el contrato con el valor V, los costos de gas del contrato Gy el valor se traduce a la otra cuenta X. Supongo que lo siguiente será cierto:

V = G + X.

Sin embargo no lo es.

Siguiendo un simple truffle initpaso, estoy trabajando con el siguiente contrato.

Pay.sol

pragma solidity ^0.4.17;

/// @title Pay - Facilitates payments.
contract Pay {
    event Payment(
        address _from,
        address _to,
        uint amount
    );

    /// @dev Makes a payment.
    /// @param _to Address to pay to.
    function pay(address _to) public payable {
        require(msg.value > 0);
        // Does this transfer the right amount of ether (msg.value measured in wei)?
        _to.transfer(msg.value);
        Payment(msg.sender, _to, msg.value);
    }
}

Y el siguiente archivo de prueba.

pay.js

var Pay = artifacts.require("./Pay.sol");

contract('Pay', function(accounts) {
    it("should put money in the first account", function() {
        return Pay.deployed().then(function(instance) {
            pay = instance;
            return web3.eth.getBalance(accounts[1]);
        }).then(function(balance){
            startingBalance = balance.toNumber();
            gasPrice = 200000;
            payAmount = 2500000;
            return pay.pay(accounts[1], {from: accounts[0], value: payAmount, gasPrice: gasPrice });
        }).then(function() {
            return pay.pay.estimateGas(accounts[1], {from: accounts[0], value: payAmount });
        }).then(function(gasCost) {
            gasSpent = gasCost * gasPrice;
            console.log(gasCost);
            console.log(gasPrice);
            console.log(gasSpent);
            return web3.eth.getBalance(accounts[1]);
        }).then(function(balance) {
            endingBalance = balance.toNumber();
            assert.equal(endingBalance - startingBalance, payAmount);
        })
    });
});

La salida es la siguiente:

  Contract: Pay
32001
200000
6400200000
    1) should put money in the first account

    Events emitted during test:
    ---------------------------

    Payment(_from: 0x627306090abab3a6e1400e9345bc60c78a8bef57, _to: 0xf17f52151ebef6c7334fad080c5704d77216b732, amount: 2500000)

    ---------------------------


  0 passing (476ms)
  1 failing

  1) Contract: Pay should put money in the first account:
     AssertionError: expected 2506752 to equal 2500000
      at test/pay.js:23:20
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:118:7)

En particular, como se ve en la salida:

V-X=-6752(¿puede incluso ser negativo?)G=6400200000

ACTUALIZACIÓN : respuesta correcta para referencia.

pay.js

var Pay = artifacts.require("./Pay.sol");

contract('Pay', function(accounts) {
    it("should put money in the first account", function() {
        return Pay.deployed().then(function(instance) {
            pay = instance;
            return web3.eth.getBalance(accounts[1]);
        }).then(function(balance){
            startingBalance = balance;
            gasPrice = 200000;
            payAmount = 2500000;
            return pay.pay(accounts[1], {from: accounts[0], value: payAmount, gasPrice: gasPrice });
        }).then(function(result) {
            gasUsed = result.receipt.gasUsed;
            return pay.pay.estimateGas(accounts[1], {from: accounts[0], value: payAmount });
        }).then(function(gasCost) {
            gasSpent = gasCost * gasPrice;
            return web3.eth.getBalance(accounts[1]);
        }).then(function(balance) {
            endingBalance = balance;
            assert.equal(endingBalance.sub(startingBalance).toNumber(), payAmount);
        })
    });

    it("should pay gas costs from the second account", function() {
        return Pay.deployed().then(function(instance) {
            pay = instance;
            return web3.eth.getBalance(accounts[0]);
        }).then(function(balance){
            startingBalance = balance;
            gasPrice = 200000;
            payAmount = 2500000;
            return pay.pay(accounts[1], {from: accounts[0], value: payAmount, gasPrice: gasPrice });
        }).then(function(result) {
            gasUsed = result.receipt.gasUsed;
            return pay.pay.estimateGas(accounts[1], {from: accounts[0], value: payAmount });
        }).then(function(gasCost) {
            gasSpent = gasCost * gasPrice;
            return web3.eth.getBalance(accounts[0]);
        }).then(function(balance) {
            endingBalance = balance;
            assert.equal(startingBalance.sub(endingBalance).toNumber(), payAmount + gasSpent);
        })
    });
});